The present invention relates to buried-heterostructure semiconductor lasers and, more particularly, to one which has desirable frequency characteristics.
Buried heterostructure semiconductor lasers have various features such as low oscillation threshold, linear injection current to light output characteristic, stable fundamental lateral mode operation and high-temperature operation capability.
A prior art double-channel planar buried-heterostructure laser diode (abbreviated as DC-PBH LD hereinafter) is shown in FIG. 1. The illustrated type of DC-PBH LD has a p-n-p-n current confinement structure thereinside which enhances current concentration to an active stripe region. Such a laser diode, using a InGaAsP system with InP substrates, shows various outstanding characteristics which are an oscillation threshold of 15-30 mA, light output larger than 50 mW, a maximum CW operable temperature of 120.degree. C., etc., each measured at a 1.3 .mu.m oscillation wavelength band. For details, reference is made to "Transactions of the Institute of Electronics and Communication Engineers of Japan, Opto and Quantum Electronics A, No. 857" or U.S. patent application Ser. No. 434,990, now issued as U.S. Pat. No. 4,252,841. Due to the internal p-n-p-n current confinement structure, DC-PBH LD does not require any stripe electrode structure for limiting a current injection region and this permits a p-side metal electrode to be formed all over the surface of a crystal prepared by epitaxial growth or a crystal surface which is entirely diffused with impurities for a higher surface concentration. Thus, the manufacturing process is simple and the yield, high. The simple manufacturing process, coupled with the excellent characteristics previously stated, enhances the reliability to such an extent that a light output of about 5 mW is stably attained even at a temperature as high as 70.degree. C.
However, a problem has been left unsolved in the DC-PBH LD concerning its high frequency response characteristic. The frequency for the light output to be halved (-3 dB) is not higher than 200-300 MHz, which means a slow response for semiconductor laser applications. The problem, as the inventors found, originates from the fact that the p-n-p-n junction constituting the current confinement structure outside the active layer region has a relatively large capacitance, which allows high frequency components of the applied current to flow through the p-n-p-n junction around the active layer and not the active layer itself. The capacitance of the p-n-p-n junction may be reduced by reducing the impurity concentration in each layer and thereby enlarging the depletion layer of the p-n junction. However, just as it helps solve the problem regarding the high frequency response, it creates various other problems. The intermediate n- and p- layers of the p-n-p-n junction, which constitute the current confining structure, are locally not thicker than 0.3-0.5 .mu.m so that the depletion layer is apt to become large enough to cause punch-through thereat. The gains of the p-n-p and n-p-n transistors of the p-n-p-n junction are increased to turn on the p-n-p-n junction. These, together with other causes, increase the proportion of current components, if DC components, which do not flow through the active layer, resulting in poor characteristics of the DC-PBH LD.